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ALEXASTDER VO^"  HTJMBOLBT.                 169
menon of the zodiacal light, is described in one part of
Humboldt^s diary, written on the voyage from Lima to
the western coasts of Mexico.    He says: " For three or
four nights, between ten and fourteen degrees northern
latitude, I saw the zodiacal light in such brilliancy as
I have never observed before.     Judging by the bril-
liancy of the stars and nebulae, the transparency of the
atmosphere in this part   of the  South  Sea is very
great.    From 14th to the 19th of March, no trace of
the zodiacal  light was visible  three  quarters   of an
hour after the sun's disk had set in the sea, although
it  was  completely dark.     An hour  after sunset,  it
suddenly became visible in great splendour, between
Aldebaran* and the Pleiades ; narrow,   long drawn
clouds were dispersed over the lovely blue near the
horizon, like before a yellow carpet; the upper ones
from time to time played into bright colours, as if it
were a second  sunset.    In this region of the firma-
ment, the  brightness  is increased   like   at the first
quarter of the moon.    About ten o'clock, the zodiacal
light was generally already very faint, and at midnight
. only a slight trace was visible/'    In our dull, so called
temperate, northern zone, the zodiacal light is plainly
visible only in the  commencement  of spring,  after
twilight, over the western horizon, and toward the end
of autumn,  before   the   morning  twilight, over   the
eastern horizon.
"We now come to Humboldt's explanation of this
extraordinary natural phenomenon, which did not
excite the attention of natural philosophers and astro-
nomers, until the middle of the seventeenth century.,
and whose views on the subject are as variously differ-
ent as imperfect. Humboldt refutes the hitherto
accepted opinion, that the luminous solar atmosphere
itself causes the zodiacal light, but considers it pro-
bable that its material cause may be the existence o
a very much flattened ring, formed of vaporous matter,
which revolves in space between the courses of Tenus
* The "beaufcifal red star in the sign of Taurus, th bull.